This invention relates generally to photomask blanks and photomasks for use in the production of semiconductors, integrated circuits (IC), large-scale integrated circuits (LSI), and similar electronic components. More specifically, this invention relates to photomasks of the type generally called hard masks and to their blanks.
In general, a photomask blank is fabricated by forming on an outer surface of a transparent substrate a thin metal film or, in place thereof, a thin film of a masking material by a process such as vapor deposition or sputtering. By applying a photolithographic process including the steps of forming a photoresist film, pattern exposure to light, and etching to this photomask blank, a thin film of the above mentioned metal or masking material is left in the form of a pattern such as a pattern of a circuit for an IC or an LSI on the transparent substrate thereby to produce a photomask.
This photomask is superposed on a semiconductor wafer and thus used for pattern transfer exposure, but since the photomask is actually an expendable article, the common practice at present is to fabricate beforehand a master mask or, further, a sub-master mask and, with the use of this master mask or sub-master mask, to fabricate masks for transferring patterns onto semiconductor wafers. For reasons of cost, photomasks having a silver emulsion film as a light-screening film are frequently used as photomasks for pattern transfer.
A circuit pattern on the above mentioned hard mask comprises a film of a material such as metallic chromium or a chromium oxide in scattered state on a glass plate. This film is not necessarily continuous over its entire expanse, there being parts existing in isolated island-like state on the glass plate. A problem arises in the following two cases as described below. The first case is that wherein this hard mask is used as a master mask or a sub-master mask and caused to contact a blank for a mask having a silver emulsion film, onto which the circuit pattern of the master mask is transferred. The second case is that wherein the circuit pattern of a mask is caused to contact a semiconductor wafer which has been coated with a resist thereby to transfer the circuit pattern.
In each of these cases, static electricity is generated by friction at the time of intimate contact and peeling off, and a discharge phenomenon occurs between the pattern parts of the hard mask. Consequently, particularly at the peripheral parts of the film comprising metallic chromium, a chromium oxide, or the like of the above mentioned island-like parts of the pattern, portions of the film are observed to have dropped off and are missing. Even small portions which are thus missing give rise to a complete loss of the function of the master mask, whereby this result is a fatal defect.
Furthermore, photomasks known in the prior art have exposed portions of the substrate outer surface. These exposed portions have lower surface strength than thin film portions and are subject to so-called staining during storage. Portions in which staining has occurred are easily damaged, in general, and lead to a lowering of the resolution of the mask.
Another problem is that, when the photomask is brought into intimate contact with a semiconductor wafer for exposure to light, the portions in which staining has occurred are very easily scratched or scored by the concavities and convexities of the semiconductor surface, dust, and other causes. Consequently, in spite of the use of the thin film of metallic chromium, chromium oxide, or the like for the light-screening film especially for their high value of surface hardness, the strength and durability of the entire hard mask is determined by those of the transparent substrate.
Still another difficulty is that, because of the accumulation of electrostatic charge during use as a result of repeated use, dust readily adheres to give rise to problems such as impairment of the intimate contact with the wafer and scratching of surfaces due to the dust.
Furthermore, together with miniaturization and denser arrangement of semiconductor devices, extremely high degrees of image quality and dimensional precision of the photomasks therefor become necessary. For this reason, in place of the ordinary photographic inspection, the electron beam inspection is coming into use. However, since photomasks after patterning are generally insulative along the expanse thereof, the parts irradiated by electron beam are charged and the accuracy of inspection is lowered to an impractical level.
In order to avoid the above difficulties, it has been proposed to use a photomask wherein an electroconductive film made of an oxide such as indium oxide and tin oxide is interposed between a transparent substrate and a masking film (U.S. Pat. No. 4,178,403). According to this mask, the above mentioned difficulties resulting from the insulative property of photomasks can be obviated. However, electroconductive photomasks of this type accompany the following other difficulties and are not practically satisfactory.
Firstly, the oxide electroconductive films as mentioned above do not have a sufficient chemical resistance and are readily attacked by a chromic acid mixture solution, a mixture solution of H.sub.2 SO.sub.4 +H.sub.2 O.sub.2 and a mixture solution of benzenesulfonic acid and a phenol which are generally used as cleaning solutions for photomasks, whereby not only the transparency of parts not covered by the masking film is considerably changed but also the oxide electroconductive films are caused to dissolve in a relatively short time and lose their conductivity. These oxide films, of course, have a sufficient resistance to organic solvent, whereas it is, as is well-known in the art, extremely difficult to remove various kinds of stains adhering to photomasks with organic solvent.
Secondly, these electroconductive oxide films have poor adhesiveness or stickiness to both glass substrates and masking films such as chromium films and are liable to form defects such as pin-holes. Furthermore, for the same reason, they generally have a poor scratch-resistance.
Because of these difficulties, this type of photomask having an oxide electro-conductive film generally fails to give an improved durability which hard masks are expected to provide.